The deployment of a full scale radio access network has always involved the installation of a considerable amount of various equipments. 2G and 3G radio access networks have historically been largely independent networks with little commonalty beyond site and antenna sharing. Traditional radio sites comprehend a number or different cabinets or units hosting transport, baseband, radiofrequency, power supply, batteries, and other equipment. For instance, a typical base station (BTS in 2G or Node-B in 3G) can have a number of different modules depending on its architecture: radiofrequency (RF) front-end module, Power Amplifier (PA) module, baseband (BB) module, and control and transmission modules. The RF modules receive/transmit signals and convert them from/to digital data, and can be split between RF front-end and high power amplifier modules. The BB module processes the signal performing multiplexing/demultiplexing and coding/decoding of the data amongst other operations allowing transmission/reception of the required data. The data is conveyed from/to the Radio Network Controller (RNC) or the Core Network (CN) through the transmission module depending on the Radio Access Network (RAN) architecture (e.g., a flat architecture would allow direct connection to the CN without any RNC for packet-switched data networks). And coordination between these functions is maintained by the control module.
The complexity of radio access network structure has made deployments slow and expensive. Now with the evolution of the technology supporting the trend to simplify the network architecture, it is becoming possible to considerably simplify the radio access network topology.
In this sense, the introduction of Remote Radio Head (RRH) technology brought the RF part of the base station closer to the antenna. The remote radio head (RRH) is a fibre-optic fed active self contained radio unit which allows the remaining elements of the base station (especially baseband units) to be remotely located from the radio head. An RF transceiver typically referred to as Remote Radio Unit (RRU), which performs all necessary RF transmit/receive functions, is installed at the top of the antenna mast or, if not possible, in the vicinity. The remaining parts, generally gathered into one or more cabinets (e.g. baseband, transmission, batteries), could be placed further away from the antenna mast, either indoor or outdoor depending on the building constraints and rental cost (generally cost outdoor are smaller, though this could vary according to the operator local context). Also the use of RRH could allow to centralise a number of base stations into a single site with fibre connections to the corresponding RRU's and antennas, which allows to save rental cost for the location of the cabinets to the expense of more complex fibre connections.
The integration of the RRU units into the antenna has been further improved by the development of the active antenna technology which basically allows all RRU functions to be hosted into the antenna. By this step forward in the technology, the antenna becomes an active element. Previously the active antenna has a passive function of radiating the signal created by the RRU and capturing the energy of the cumulated signals from the terminals in the network. Now the active antenna has an architecture design combining an active device into a part of a passive element. Typically this is achieved by using a number of synchronous RF modules each one transmitting and receiving part of the overall signal, which can be seen as decomposing the high power signal into a number of lower power signals (for example 10 modules of 4 W allowing to build an overall 40 W output power output). [“Active antenna elements for millimeter-wave cellular communications”, M. J. Vaughan, W. Wright, R. C. Compton, Signals, Systems, and Electronics, ISSSE '95, URSI International Symposium, 1995].
In addition to this, the introduction of the flat architecture for the packet switching (PS) domain also allowed further simplification of the RAN architecture with the integration of RNC functionalities in the BTS/Node B and Core Network (no BSC/RNC nodes required in the network), whilst historically BSC/RNC's were typically deployed additionally to base stations. This flat architecture, also called Collapsed Architecture because many RNC functions are directly collapsed into the Node B, is based on eHSPA (evolved High Speed Packet Access), defined by the 3GPP release 7 specifications relying on HSPA (High-Speed Downlink Packet Access) and HSUPA (High-Speed Uplink Packet Access) for the 3G data bearers over the air.
FIG. 1 illustrates an example of a typical compact design for a BS site using conventional passive antennas (10, 10′, 10″); typically three antennas are used, i.e. one per sector, and a classical cabinet (11) to house a sheltered baseband, transmission, radio, power supply and battery equipment. A typical outdoor base station with a typical configuration has a large footprint and can weight several hundred kilos.
Still, with current RAN architecture, despite the improvement brought by RRH technology (versus classical base station) on each site in addition to the passive antennas, fibre optics connection to each RRU (3 in total, i.e. one RRU per sector) and the self-contained cabinet of the BTS/Node B to host the baseband/transport and potential power supply and battery solutions are still needed. Therefore a breakthrough in terms of the simplification of the site infrastructure is highly desirable in order to ease and speed-up site deployment at a lower operating cost.